The present invention relates generally to a method and apparatus for the thermal treatment of particulate materials with the gaseous or gassifiable fuels in air in which various combinations of the following methods and apparatus are employed: a partially or wholly briquetted fuel; a partially or wholly briquetted charge; a cascading process path; a path with a varying process cross section; an annular process path; computer control of the rate of the input of the charge, the fuel and injected air; and/or charge sorted in order to limit the range of the size of the particles of the charge.
Three major areas in the lime production industry present challenges and opportunities for improvement: production costs and efficiency; production quality, especially as it relates to the steel industry; and hazardous waste disposal. Lime production involves the process of calcination: lime is derived from limestone by heating the limestone above the equilibrium temperature of calcium carbonate so that it dissociates into calcium oxide (lime) and carbon dioxide. EQU CaCO.sub.3 CaO+CO.sub.2
This process of calcination refers to a class of reactions which involves heating a substance until a weight gain or loss reaction occurs without melting the original substance. The calcination of limestone is the representative of the weight loss category. The heat energy required for this reaction is supplied by fuel combustion, usually coal; but oil, natural gas and other fuels such as wood and old tires are also used.
Substantial energy is required for calcination. The theoretical heat of reaction at 900.degree. C. (1652.degree. F.) is more than 720 Kcal per Kg of CaO produced. Therefore 2,400,000 BTU's or roughly 400 pounds of coal are required to calcinate a ton of lime.
In a counter-flow, shaft or vertical type of kiln, the limestone is introduced at the top of the kiln and makes its way slowly down through the device, finally being discharged at the bottom as lime. Air is introduced at the bottom of the shaft and is drawn upward, counter-flow to the descending limestone/lime particles. As the air moves upward from the bottom of the shaft it is warmed by extracting heat from the lime which is about to exit the kiln. Thus in the lower portion of the kiln the lime is tranfering heat to the air, the heat transfer is from the solids to the gas. At the point, part way up in the shaft, where fuel is introduced into the system the heat transfer is reversed. Due to the energy release of the burning fuel the air becomes hotter than the limestone/lime particles and heat transfer then takes place from the kiln gases to the solids. Thus in the mid to upper region of the shaft the heat transfer is from the gas to the solids.
In recent times the mixed feed kiln has been restricted to using coke and/or petroleum coke with little or no volatile fraction. The shaft kiln of modern times (the last century) has used a liquid or gaseous fuel introduced at a mid-point so as to utilize the volatile fraction of the fuel. This results in gas temperatures where the fuel is introduced into the kiln which are on the order of 2800 to 3000 F.
At the on-set of calcination the reaction rate is at its maximum, thus the heat flux density in the kiln should be also, but the temperatures required for calcination (even at a relatively high rate) never exceed 2000 to 2200 F. It is necessary, however, to raise the temperature of the limestone from ambient when it is introduced, to calcination level (about 1652.degree. F. or so).
Fuel has jumped from less than 10% of the cost of producing lime in the early 1970's to more than a third, with most modern lime producers using coal as a fuel. However, the conditions necessary for complete combustion of fossil fuels are less than ideal for the calcination of limestone. The adiabatic flame temperature of coal with stoichiometric air is in the range of 1650.degree. C. (3000.degree. F.), whereas the calcination of limestone in an atmosphere of 25 to 35% CO.sub.2 (flue gas composition) only requires 950.degree. C. (about 1750.degree. F.). The actual heat used by most commercial kilns is 1.5 to 5 times the theoretical requirement. The excessively high temperatures waste heat, place an undue burden on the kiln design and refractory selection of having to insulate the combustion region against temperatures nearly 1,000.degree. F. higher than necessary. The high temperatures in ordinary kilns also tend to minimize the specific surface area of the lime that is produced; higher specific surface areas promote the rate at which lime reacts with water as well as other substances and is becoming an increasingly more important property of lime.
Utilization of the quarried or mined limestone as a feed-stock to the kilns is an important criteria in the overall cost structure of a lime plant. The quarrying and mining operations yield limestone that ranges in size from boulders weighing several tons to dust-sized particles. The lime kilns, however, cannot tolerate such a wide size range, thus it is necessary to crush and screen the limestone to some narrower size range before it can be introduced into a lime kiln. There are limits to both the largest and smallest pieces that can effectively be used in a kiln, while the over-size pieces can be crushed down to fit into the desired size range the under-size (called fines) material becomes a by-product. Although there is a limited market for the fines the value added is trivial or non-existent compared to being able to use the limestone as a feed-stock to a kiln. Consequently, the producer is driven to use as wide a size range as possible in an effort to maximize the utilization of the quarried limestone. Smooth, efficient operation of a lime kiln, however, requires as narrow a size range as possible. The ideal kiln feed-stock would be uniform spheres. These two competing criteria for sizing the limestone to be used as a kiln feed result in a compromise that is never wholly satisfactory to either demand.
Normal run-of-quarry (or mine) limestone is sent to a secondary crushing and screening operation where the material is reduced to kiln-feed size and the undersize limestone is set aside. Because the kiln-feed is then delivered to the kiln as a lump aggregate of many sized pieces there are sharp limits to the range of size that any given kiln can process. Usually acceptable practice in the industry limits the size range to about a 2:1 ratio, the largest stone is only twice the size of the smallest. Most lime plants actually operate with a size range closer to 3:1, particularly if they have but one or two kilns, in an effort to keep the quarry yield within economic bounds. A typical secondary crushing and screening unit recycles the over-size stone, rejects the undersize and sends the kiln-feed size range to a stockpile. As the kiln-feed size range is constricted the portion of the quarry output that is lost as fines or undersized limestone increases dramatically. However, the new fuel efficient shaft kilns and rotary kilns with pre-heaters cannot accept a wide size range of feed nor a feed with a very small bottom size in the feed.
This results in a considerable loss of quarry production as undersize material; the overall quarry to product yield in a plant with only one kiln or single feed size is rarely a great deal better than 3:1 and may be as poor as 6:1.
The product, lime in the form of pebbles, has two important deficiencies when used in the production of steel. First, it is soft and readily breaks down in the material handling equipment used to move and store it; and second, when introduced into the steel hot metal bath, a coating of calcium silicate compounds forms around the pebble which extends the time necessary to process a given batch of hot metal. Modern steel making operations typically use two types of lime: high calcium lime which is more than 90% CaO; and dolo-lime, from calcining dolomite, a sedimentary rock comprised of an equal molar ratio of calcium and magnesium carbonates. A mix of oxides balances with the chemistry of the refractories in the steel making vessel and extends the useful life of the refractory lining.
Competition in the steel industry has led to increased efforts in the direction of quality control, and economy of operations. The breakage of lime in handling and transport results in a product that is not uniform in size, nor is the size distribution predictable. When lime is added to steel making vessels the fine fraction of the lime is carried away into the dust collection system of the steel plant. The amount of lime actually added to the hot metal is thus a combination of the amount of the lime charged and the size distribution of the lime. While control of the size distribution is not practical, the steel producers have specified that the lime will have a certain minimum size thus eliminating, or at least minimizing, the fines in the lime charge. This makes the amount of lime charged to the hot metal bath more consistent and thus more predictable.
The use of two types of lime, high calcium and dolo-lime, simply doubles the number of problems associated with the feed and control of additives to the steel making process.
The demands of the steel producer for lime of a known and consistent size distribution, free of fines, creates significant problems for the lime manufacturer. A typical lime manufacturing plant quarries or mines about 3 tons of limestone for each ton of lime that is produced. Very small pieces of limestone, that which would pass a 4 Mesh screen, are not readily utilized in modern calcining kilns. The fine limestone is usually sold in the crushed stone or gravel market. Additional breakage occurs during manufacturing and product handling as the lime is being loaded for transport. Fine lime, passing a 6 or 8 Mesh screen, becomes something of a marginal product. Problems of dust, housekeeping, and handling a fine material makes the lime fines less desireable even though the end use may be to dissolve the lime into a slag bath. The overall effect is to increase the material cost of making lime, and make it more difficult to improve the efficiencies of lime manufacturing.
A number of lime producers have and do make a product in which an iron source such as mill scale, is combined with limestone and the two are processed together. This results in a lime/calcium ferrite material of variable composition, and one which poses considerable processing problems. Lime is readily fluxed by iron to yield low melting eutectic compositions. In kilns where the product is exposed to a flame there is the potential to slag or fuse the product to the refractory walls of the kiln. The variable composition is a result of the limestone of different sizes contacting different amounts of iron as it passes through the kiln. Small pieces of limestone have a proportionately higher surface area than large pieces and therefore have a greater contact with the iron, resulting in an iron richer composition than larger pieces.
Amendments in 1984 to the Resource Conservation & Recovery Act (RCRA) will make it necessary to treat all hazardous wastes by 1990. It will no longer be possible to dispose of unstabilized, hazardous liquid wastes. Permitted landfill sites are becoming more restrictive as EPA moves to close unregulated dumps (about 500 landfill sites per year are being shut down by EPA). Incineration is considered to be an acceptable disposal technique, but costly in its present method of operation.
Most existing incinerators have a support flame to ensure that the combustor is sufficiently hot to effectively destruct the hazardous waste irrespective of its calorific content. Existing incinerators also either waste the heat that is produced by the combustion process or recover some of it as steam. The newest designs recover the heat as steam for power (electric) generation.